1. Field of the Invention
The present invention relates to a spark plug mountable in an internal combustion engine and configured to ignite an air-fuel mixture.
2. Description of the Related Art
A spark plug for ignition has been used in an internal combustion engine. A spark plug generally includes a center electrode, an insulator holding the center electrode in an axial hole, a metal shell surrounding and holding the radial periphery of the insulator, a ground electrode having an end joined to the metal shell and another end defining a spark discharge gap between the center electrode and the ground electrode. A spark discharge is carried out between the center electrode and the ground electrode, thus igniting an air-fuel mixture. In the spark plug, to prevent the wear and tear of the electrodes resulting from the spark discharge, an electrode tip containing a noble metal as a main component is joined to at least one of the center electrode and the ground electrode (see JP-A-5-159853, for example).
In the spark plug described in JP-A-5-159853, a hole is bored in a leading end surface of the center electrode; the electrode tip is fitted thereto; and thereafter the electrode tip and the center electrode are welded together. A metallic core (core member) having a high thermal conductance, such as copper or silver, is embedded in the center electrode, and the core member contacts or is located close to the electrode tip fitted to the hole. The core member can efficiently improve the heat dissipation at a leading end portion of the center electrode.
However, when the diameter of the spark plug is reduced so as to secure the degree of freedom of engine design in order to heighten the output of an automobile engine or to save fuel cost, the diameter of the center electrode will also be reduced. In turn, the cross-sectional area of the core member will become small. Therefore, the heat dissipation property may be lowered. Further, JP-A-5-159853 requires an extra process for forming a hole, to which the electrode tip is fitted in the leading end surface of the center electrode. If the electrode tip and the center electrode are joined together without forming this hole, due to a decrease in heat dissipation property as mentioned above, the heat dissipation property of a molten bond formed by welding these two elements will not be sufficient. Consequently, the molten bond may oxidize at a high temperature when the engine is heavily loaded, and hence the joining capability of the electrode tip may be lowered.